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REPRODUCTIVE ISOLATING MECHANISMSThe biological definition of a species is a group of similar organisms that can interbreed to produce fertile, viable offspring. Some extend this to say that this reproduction must occur under natural, not artificial (e.g., in captivity) situations. When an ancestral species gives rise to two new species, what determines whether the two new species can reproduce? Consider the mechanisms that restrict gene flow: reproductive isolating mechanisms. PREZYGOTIC ISOLATING MECHANISMS prevent the formation of viable zygotes. ISOLATING MECHANISMSPREZYGOTIC ISOLATIONPOST ZYGOTIC ISOLATIONPrezygotic isolating mechanisms lead to reproductive isolation by preventing the formation of hybrid zygotes.Postzygotic isolating mechanisms are those in which hybrid zygotes fail to develop or develop abnormally, or in which hybrids cannot become established in nature.

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Red-legged Frog (Rana aurora) Bull frogs (Rana catesbiana)1. Ecological IsolationThe geographic ranges of two species overlap, but their ecological needs or breeding requirements differ enough to cause reproductive isolation. Habitat isolation: breed in different habitats within cruising range. Allochronic isolation: breed at different times.Pollinator (floral) isolation: different insect species or different individuals.In central and northern California, Red-legged Frog (Rana aurora) breeds in fast-moving, ephemeral streams. Artificially introduced Bullfrogs (Rana catesbiana) breed in permanent ponds. The metamorphosis times of the two species' tadpoles are correspondingly different. 2. Temporal IsolationTwo species whose ranges overlap have different periods of sexual activity or breeding seasons.E.g.. 1. Red-legged Frog (Rana aurora) breeding season lasts from January to March. The closely related Yellow-legged Frog (Rana boylii) breeds from late March to May.

2. Drosophila persimilis breeds in early morning, while closely related Drosophila pseudoobscura breeds in the afternoon.

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D. persimilisD. pseudoobscura &3. Behavioral / Ethological IsolationSpecies with complex courtship rituals (breeding calls, mating dances, etc.) usually exhibit a stereotyped "give-and-take" between male and female before actual mating takes place.

These Galpagos blue-footed boobies select their mates only after an elaborate courtship display. This male is lifting his feet in a ritualized high step that shows off his bright blue feet. The display behavior of other species of boobies, some of which also occur in the Galpagos, is much different.4. Mechanical / Morphological IsolationDifferences in the external genitalia or morphological size of the individuals.eg shell coiling5. Gametic / Physiological IsolationIn this case, sperm and ova of the two species are chemically (genetically) incompatible, and will not fuse to form a zygote, and if a zygote is formed they will not be viable.

Giant Red Urchin (Strongylocentrotus franciscanus)& Purple Urchin (Strongylocentrotus purpuratus) co-inhabit the rocky intertidal along western U.S., but they do not interbreed. Their gametes are genetically / chemically incompatible, maintaining species integrity.

6. Gametic / Physiological IsolationWhen there is interspecific mating, the gametes are killed in some species by the chemicals present in their genital tracts. E.g.: Bufo fowleri & Bufo valliceps., Drosophila.7. Cytological Isolation When there is interspecific mating, the isolating mechanism operates at the level of fertilization. Fertilization doesnt take place successfully, due to the difference in the number of chromosomes between the two individuals.POST ZYGOTIC ISOLATIONPost zygotic mechanisms prevent hybrids from passing on their genes. 1. Hybrid InviabilityA zygote may be formed by the union of sperm and egg from the two species, but the embryo dies after a few cell divisions. The genetic information from male and female parents is insufficient to carry the organism through morphogenesis. E.g. cross between goat and sheep produces normal embryos, but they die much before birth.2. Hybrid SterilityViable hybrid is produced (often physically more vigorous than either parent), but is unable to reproduce due to meiotic problems. Example: Horse (Equus caballus) x Donkey (Equus asinus) = Mule or Hinny3. Hybrid BreakdownSuccessive generations of hybrids suffer greatly lowered fertility --> sterility. Eventually, they are selected out of the population. Example: D. pseudoobscura X D. persimilisHybrid males are sterile, hybrid females are fertile, but exhibits very low general viability when back crossed to one of its parents, in comparison to its parental sps.

ORGIN OF REPRODUCTIVE ISOLATION

Bateson-Dobzhansky-Muller Model

Model of the evolution of genetic incompatibility. Theory was first described by William Bateson in 1909, then independently described by Dobzhansky in 1934, and later elaborated by Herman Muller.Explains how incompatibilities between closely related species develop without either of them going through an adaptive valley. In its simplest form the model shows that changes in at least two loci are required to cause hybrid incompatibility. This is based on the idea that a new allele which has arisen at one locus in one population should not cause decreased fitness when placed into the identical (except for one allele) genetic background of the second population. Therefore another allele at a second locus which is incompatible with the first must have arisen.It also explains how when new alleles arise they can be passed down in the newly formed species. Theoretically, an individual that would have a mutation at the chromosomal level would not be able to reproduce with any other individual.

So how is it that a brand new lineage can arise if there is only one individual with that mutation? The Dobzhansky-Muller Model attempts to theorize how this could happen!!

In their explanation, a new allele arises and becomes fixed at one point. In the second newly diverged lineage, a different allele arises at a different point on the gene. The two diverged species are now incompatible with each other because they have two alleles that have never been together in the same population. This changes the proteins that are produced during transcription and translation, which could make the hybrid offspring inviable. However, each lineage can still hypothetically reproduce with the ancestral population. If these new mutations in the lineages are advantageous, eventually they will become permanent alleles in each population and the ancestral population has split into two new species.

The Dobzhansky-Muller Model is also able to explain how this may happen at a large level with whole chromosomes. It is possible that over time during evolution, two smaller chromosomes may undergo centric fusion and become one large chromosome. If this happens, the new lineage with the larger chromosomes is no longer compatible with the other lineage and hybrids cannot happen.